This invention relates to a dual mode solid state power switch which operates as a simple saturated switch when the load current is less than a predetermined value, and as a Darlington circuit when an overload or transient condition exists, such power switch being referred to hereinafter as a power switch of the type described.
A power switch of the type described is disclosed in U.S. Pat. No. 3,678,291 issued July 18, 1972 to R.J. Coe. In this power switch, a two-stage transistor amplifier is interconnected in a Darlington configuration with a diode connecting the collectors of the two stages. When the load current is less than a predetermined value, the output transistor operates as a saturated switch with base drive current for this transistor flowing through the emitter-base junction of the first stage. In this condition, the diode is reverse-biased preventing the flow of collector current in the first stage. As the load current exceeds said predetermined value, the voltage at the anode of the diode increases while the voltage at the cathode is lowered due to the increased load current flowing in the output stage. Ultimately, the diode is forwardly biased shifting the mode of operation of the power switch from a saturated switch mode to a Darlington circuit mode. In the latter mode, the output stage is capable of handling the overload current without burning out.
While the Coe power switch efficiently handles rated load current while operating in its saturated switch mode, its efficiency is reduced when operating in its Darlington circuit mode under overload conditions due to the flow of collector current in the first stage. While such current is returned to the load, it passes through the diode whose forward resistance introduces significant losses that reduce the efficiency of the switch.
It is to improve the efficiency of a power switch of the type described when operating in the Darlington circuit mode that constitutes an object of the present invention.